Thermodynamics of Formation of Coffinite, Usio4

Thermodynamics of formation of coffinite, USiO4

Xiaofeng Guoa,b, Stéphanie Szenknectc, Adel Mesbahc, Sabrina Labsd, Nicolas Clavierc, Christophe Poinssote, Sergey V. Ushakova, Hildegard Curtiusd, Dirk Bosbachd, Rodney C. Ewingf, Peter C. Burnsg, Nicolas Dacheuxc, and Alexandra Navrotskya,1

aPeter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616; bEarth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545; cInstitut de Chimie Séparative de Marcoule, UMR 5257, CNRS/CEA/Université Montpellier/Ecole Nationale Supérieure de Chimie de Montpellier, Site de Marcoule, 30207 Bagnols sur Cèze, France; dInstitute of Energy and Climate Research, Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; eCEA, Nuclear Energy Division, RadioChemistry & Processes Department, BP 17171, 30207 Bagnols sur Cèze, France; fDepartment of Geological and Environmental Sciences, School of Earth Sciences, Stanford University, Stanford, CA 94305; and gDepartment of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN 46556

Contributed by Alexandra Navrotsky, April 20, 2015 (sent for review February 10, 2015)

Coffinite, USiO4, is an important U(IV) mineral, but its thermody- The precipitation of USiO4 as a secondary phase should be namic properties are not well-constrained. In this work, two dif- favored in contact with silica-rich groundwater (21) [silica − ferent coffinite samples were synthesized under hydrothermal concentration >10 4 mol/L (22, 23)]. Natural coffinite samples conditions and purified from a mixture of products. The enthalpy are often fine-grained (4, 5, 8, 11, 13, 15, 24), due to the long of formation was obtained by high-temperature oxide melt solu- exposure to alpha-decay event irradiation (4, 6, 25, 26) and are tion calorimetry. Coffinite is energetically metastable with respect associated with other minerals and organic matter (6, 8, 12, 18, ± to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 3.9 kJ/mol. 27, 28). Hence the determination of accurate thermodynamic Its standard enthalpy of formation from the elements at 25 °C is data from natural samples is not straightforward. However, the −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was char- synthesis of pure coffinite also has challenges. It appears not to acterized by X-ray diffraction and by thermogravimetry and differen- form by reacting the oxides under dry high-temperature condi- tial scanning calorimetry coupled with mass spectrometric analysis tions (24, 29). Synthesis from aqueous solutions usually produces of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 UO and amorphous SiO impurities, with coffinite sometimes 2 2 CHEMISTRY starting around 450 °C in air and thus has poor thermal stability in the being only a minor phase (24, 30–35). It is not clear whether ambient environment. The energetic metastability explains why cof- these difficulties arise from kinetic factors (slow reaction rates) finite cannot be synthesized directly from uraninite and quartz but or reflect intrinsic thermodynamic instability (33). Thus, there canbemadebylow-temperatureprecipitation in aqueous and hy- are only a few reported estimates of thermodynamic properties drothermal environments. These thermochemical constraints are in of coffinite (22, 36–40) and some of them are inconsistent. To accord with observations of the occurrence of coffinite in nature resolve these uncertainties, we directly investigated the ener- and are relevant to spent nuclear fuel corrosion. getics of synthetic coffinite by high-temperature oxide melt solution calorimetry to obtain a reliable enthalpy of formation and uranium | coffinite | USiO4 | U(IV) minerals | calorimetry explored its thermal decomposition. Results and Discussion n many countries with nuclear energy programs, spent nuclear Ifuel (SNF) and/or vitrified high-level radioactive waste will be We used two independently prepared coffinite samples. A phase- disposed in an underground geological repository. Demonstrat- pure coffinite sample prepared at Institut de Chimie Séparative ing the long-term (106–109 y) safety of such a repository system is de Marcoule (ICSM), France is labeled “coffinite-F” and a less a major challenge. The potential release of radionuclides into pure material prepared at Forschungszentrum Jülich, Germany is the environment strongly depends on the availability of water and the subsequent corrosion of the waste form as well as the Significance formation of secondary phases, which control the radionuclide solubility. Coffinite (1), USiO4, is expected to be an important Coffinite, USiO4, is an important alteration mineral of uraninite. alteration product of SNF in contact with silica-enriched ground- Its somewhat unexpected formation and persistence in a large water under reducing conditions (2–8). It is also found, accompa- variety of natural and contaminated low-temperature aqueous nied by thorium orthosilicate and uranothorite, in igneous and settings must be governed by its thermodynamic properties, metamorphic rocks and ore minerals from uranium and thorium which, at present, are poorly constrained. We report direct calo- sedimentary deposits (2, 4, 5, 8–16). Under reducing conditions rimetric measurements of the enthalpy of formation of coffinite. The calorimetric data confirm the thermodynamic metastability in the repository system, the uranium solubility (very low) in of coffinite with respect to uraninite plus quartz but show that it aqueous solutions is typically derived from the solubility product can form from silica-rich aqueous solutions in contact with dis- of UO2. Stable U(IV) minerals, which could form as secondary solved uranium species in a reducing environment. These con- phases, would impart lower uranium solubility to such systems. straints on thermodynamic properties support that coffinitization Thus, knowledge of coffinite thermodynamics is needed to con- in uranium deposits and spent nuclear fuel occurs through disso-

strain the solubility of U(IV) in natural environments and would lution of UO2 (often forming hexavalent uranium intermediates) be useful in repository assessment. followed by reaction with silica-rich fluids. In natural uranium deposits such as Oklo (Gabon) (4, 7, 11, 12, 14, 17, 18) and Cigar Lake (Canada) (5, 13, 15), coffinite has Author contributions: X.G., S.S., A.M., S.L., N.C., C.P., S.V.U., H.C., D.B., N.D., and A.N. been suggested to coexist with uraninite, based on electron probe designed research; X.G., S.S., A.M., and S.L. performed research; X.G., S.S., A.M., S.L., N.C., C.P., S.V.U., H.C., D.B., R.C.E., P.C.B., N.D., and A.N. analyzed data; and X.G., S.S., microanalysis (EPMA) (4, 5, 7, 11, 13, 17, 19, 20) and trans- A.M., S.L., N.C., C.P., S.V.U., H.C., D.B., R.C.E., P.C.B., N.D., and A.N. wrote the paper. mission electron microscopy (TEM) (8, 15). However, it is not The authors declare no conflict of interest. clear whether such apparent replacement of uraninite by a cof- 1To whom correspondence should be addressed. Email: [email protected]. finite-like phase is a direct solid-state process or occurs mediated This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. by dissolution and reprecipitation. 1073/pnas.1507441112/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1507441112 PNAS Early Edition | 1of5 Downloaded by guest on September 27, 2021 labeled “coffinite-G.” In both cases, considerable purification after initial synthesis was done to separate impurities (SI Appendix). The purpose of measuring and comparing both samples was to test whether consistent thermochemical data could be obtained on materials prepared and purified independently in different laboratories. Before calorimetric measurements, samples were characterized by TEM, infrared spectroscopy (IR), powder X-ray diffraction (XRD), simultaneous differential scanning calorimetry and ther- mogravimetric analysis (DSC-TG) coupled with mass spectrometric analysis of evolved gases (MS-EGA), and by EPMA. The details are given in Experimental Methods and in SI Appendix. The XRD patterns (SI Appendix, Fig. S1) show the reflections expected for a zircon-type structure (space group I41/amd). Lattice parameters are a = b = 6.983(3) Å and c = 6.263(4) Å for coffinite-G, and a = b = 6.990(1) Å and c = 6.261(1) Å for coffinite-F. Crystallite size was estimated from X-ray peak broad- ening (65 nm for coffinite-G and 85 nm for coffinite-F). This observation is consistent with previous reports (8, 24, 26, 34, 40). Although the particle size may affect the thermodynamic properties, we did not investigate this further. Even though coarse- grained but impure coffinite (>10 μm) has been documented from the Grants uranium region, New Mexico (8, 41), Colorado (8, 10), and hydrothermal deposits in Czech Republic (42), natural coffinite and materials produced in SNF alteration are usually fine-grained, having similar particle size as our synthetic samples. So, the data obtained here are applicable to most “real world” situations. TG (Fig. 1) indicates a negligible amount of water in sample coffinite-F, not quantifiable from the MS trace. This observation agrees with Raman spectra on the same sample which show no signal associated with water (43). The water content of coffinite-G quantified by MS-EGA is 0.37 mol H2O per mole of USiO4 (slightly less than that estimated from TG analysis: 0.43 mol H2O per mole of USiO4). The water signal from MS suggests that there may be two water bonding sites, one with release near 150 °C and the other around 275 °C. The second peak (Fig. 1C)hasahigh- temperature tail up to 450 °C, suggesting some water molecules are strongly interacting with the sample. Above 450 °C, both samples oxidize and slowly decompose in air, as indicated by weight increase on the TG trace due to oxidation of U(IV). This is consistent with previous reports (24, 30). After heating to 1,000 °C in air, no trace of USiO4 is detected in either sample. Coffinite-F decomposes to U3O8 and amorphous SiO2 (SI Appendix,Fig.S2), and coffinite-G decomposes to a mixture of crystalline and amorphous SiO2 phases and a uranium- rich phase (possibly containing some silica and whose XRD pattern resembles that of U3O8). EPMA of coffinite-F (SI Appendix, Table S1) confirms USiO4 stoichiometry. The X-ray map (SI Appendix,Fig.S3) and back- scattered electron image show that coffinite-F is pure and homo- geneous with no detectable secondary phases. As for coffinite-G, the Si/U ratio of this sample is 4.19 and its chemical composition is USiO4·3.19SiO2·0.37H2O, obtained by combining EPMA result Fig. 1. DSC-TG of coffinite-F [DSC trace in bold solid, and TG trace in bold + and water content from MS, described in detail in SI Appendix. dash (A)] and coffinite-G (B) with corresponding H2O (m/z +18) trace from This bulk composition is used to interpret the calorimetric data. MS-EGA (C). Through thermochemical cycles (SI Appendix, Tables S2 and S3), the enthalpy of formation (ΔH ) of coffinite-F from binary f,ox SI Appendix oxides (uraninite plus quartz) is 24.6 ± 3.1 kJ/mol, obtained from crystalline silica ( , Fig. S4), and was corrected by calorimetry in sodium molybdate (3Na O·4MoO ); and 26.7 ± using the drop solution enthalpy of silica glass. The correction 2 3 “ ” 4.7 kJ/mol, derived from calorimetry in lead borate (2PbO·B O ) for excess water is less simple. If the water is free water and 2 3 ΔH (Table 1). Using independent measurements in two different thus weakly bonded to the sample, the corrected f,ox of solvents confirms consistency and indicates that there were no coffinite is −5.5 ± 3.5 kJ/mol (SI Appendix,TableS4). However, it unanticipated problems in calorimetric procedures. Details of is unlikely the H2Oisfreewater,asIRshowswatersignalforthe calorimetry are given in SI Appendix. sample dried at 200 °C for 48 h (SI Appendix,Fig.S6), and MS Due to limited sample amount, calorimetry of coffinite-G was shows that the water is removed gradually up to 450 °C, which performed only in 3Na2O·4MoO3 solvent. Correction for excess suggests that at least part of the H2O in this sample is strongly silica and water was required to derive ΔHf,ox of coffinite from bonded. Assuming water in coffinite is adsorbed on the surface this sample. The excess Si is present as amorphous to poorly with an integral adsorption enthalpy of −80 kJ/mol per mole of

2of5 | www.pnas.org/cgi/doi/10.1073/pnas.1507441112 Guo et al. Downloaded by guest on September 27, 2021 Table 1. Measured enthalpies of drop solution and enthalpies Langmuir (22) or ΔHf,ox = 4.3 ± 5.6 kJ/mol collected in the book of formation from oxides edited by Grenthe (37) and suggest that the estimation by Szen- knect et al. (40) is more reliable than those values (Table 2). Source ΔHds, kJ/mol ΔHf,ox, kJ/mol The substantially positive ΔHf,ox also explains why coffinite Coffinite-F −121.43 ± 1.54* 24.6 ± 3.1 cannot be formed directly from UO and SiO and agrees with † 2 2 −102.01 ± 3.10 26.7 ± 4.7 the observation that coffinite decomposes upon heating to a Coffinite-G 44.14 ± 0.94* (−5.5 ± 3.5) moderate temperature as seen from DSC-TG experiments. ‡ 44.14 ± 0.94* 24.1 ± 3.5 Coffinite metastability was also inferred by Costin et al. (33) in 44.39 ± 0.94* 23.9 ± 4.0§ hydrothermal synthesis. They noted that the dissolution–reprecipitation process slows toward the coffinite end of the Th1−xUxSiO4 *Measured in 3Na2O·4MoO3 solvent. † – Measured in 2PbO·B2O3 solvent. series, forming a correspondingly increasing amount of a Th U ‡This value obtained from a correction considering the water was strongly dioxide phase. As a result, the formation of a uranothorite phase bonded and its interaction enthalpy was assumed to be −80 kJ/mol. with x > 0.26 (coffinite included) is suggested to be thermody- §This value obtained from a correction considering metaschoepite as an namically unfavorable (40). In addition, a Th–U dioxide phase and impurity phase containing all of the water. amorphous silica were inevitably present in these products (24, 30). Because coffinite is metastable with respect to uraninite plus quartz, why can it form and persist widely in uranium ore de- H O, similar to the values observed for water adsorption on 2 posits (2, 12, 19)? First, one should realize that coffinite can be alumina and titania (44–46) and, through proper thermochemi- stable with respect to aqueous species over a wide range of cal cycles (SI Appendix, Table S4), a value of ΔHf,ox of coffinite ± concentrations. Langmuir (22) assumed the average silica con- (24.1 3.5 kJ/mol) is obtained, in agreement with the results for −3 coffinite-F (Table 1). centration to be 10 M in the solution where the aqueous The tightly bound water could originate from metaschoepite equilibrium between coffinite and uraninite is established (5): USiO4(s) + 2H2O(l) ⇄ UO2(s) + Si(OH)4(aq). A calculation based (UO3·2H2O), which could be formed from partial oxidation of U(IV) (47) that originally was not incorporated in the coffinite on Gibbs free energy obtained from solubility experiments done structure, but rather was embedded in the gelatinous layer of by Szenknect et al. (40) and auxiliary data (37) gives the Gibbs ± excess amorphous silica (33). This phase may be hard to detect free energy of this reaction to be 5.7 5.8 kJ/mol, which is es- by XRD, especially if it is fine-grained or poorly crystalline. If all sentially zero. Thus, their analysis suggests that coffinite can CHEMISTRY of the water is in metaschoepite, the composition of coffinite-G form from aqueous U(IV) in contact with silica-rich aqueous solutions, even though it is metastable with respect to crystalline can be written as 0.815(USiO4)·3.379SiO2·0.185(UO3·2H2O). Making this correction through an appropriate thermochemical UO2 plus SiO2. Our enthalpy data support this general conclusion. Therefore, coffinite can be an alteration product of UO cycle (SI Appendix,TableS5) gives the corrected ΔHf,ox of USiO4 2 as 23.9 ± 4.0 kJ/mol, again in agreement with the value for under repository, hydrothermal, metamorphic, or even igneous coffinite-F. conditions as long as its formation can proceed through pre- Thus, our measurements indicate that coffinite is energetically cipitation from aqueous solution. metastable with respect to uraninite plus quartz by about 25 The presence of water may play an additional significant role kJ/mol. Coffinite is also energetically metastable with respect to in stabilizing the coffinite phase or favoring the coffinitization mixture of UO2 and amorphous SiO2, as the enthalpy difference process (4). Deditius et al. (8) studied the composition of natural between quartz and glass is 9.1 kJ/mol (48). For the Gibbs free coffinite and found it can crystallize with variable amounts of energy of coffinite formation from uraninite plus quartz to be H2O apparently incorporated in the material. However, whether negative at 25 °C, the entropy of formation must be strongly this water is truly in the coffinite structure (24, 49), is associated positive and probably unreasonably large for a solid-state reaction. with the excess silica (24, 43), or represents a fine intergrowth of Thus, we conclude that coffinite is thermodynamically metastable coffinite and some other phase, e.g., metaschoepite, is not clear. relative to uraninite plus quartz under ambient conditions. This Although it is plausible that coffinite forms as an alteration conclusion is in good agreement with the solubility experiments product from interaction of uraninite with Si-rich fluids, there and calculations of Szenknect et al. (40) using reference entropy may also be an alternative explanation for its formation. Con- data (22, 40) and obtaining ΔHf,ox = 10 ± 32 kJ/mol (Table 2). The sider underground nuclear waste repositories, ore deposits, or present values disagree with ΔHf,ox = -5.8 kJ/mol estimated by natural reactors where the high alpha dose (6, 25, 50) triggers

Table 2. Comparison of thermodynamics of formation of coffinite at 25 °C obtained in different studies −1 −1† Source ΔHf,ox, kJ/mol* ΔGf,ox, kJ/mol* ΔSf,ox, J mol ·K ΔH°f, kJ/mol ΔG°f, kJ/mol

Langmuir (22) −5.7‡ 5.6 −37.9 −2,001.3 −1,882.4 ‡ Grenthe et al. (37) 4.3 ± 5.6 4.4 ± 4.2 −0.4 ± 12.8 −1,991.3 ± 5.4 −1,883.6 ± 4.0 Hemingway (36) 2 ± 6 −1,886 ± 6 Szenknect (40) −105 ± 32‡ 16 ± 6 −407 ± 56 −2,101 ± 32 −1,872 ± 6 10 ± 32‡,§ −19.1 ± 12.8§ Fleche (39) 82 −1,913 ‡ Coffinite-F 24.6 ± 3.1 −1,971.0 ± 3.4 26.7 ± 4.7 −1,968.9 ± 4.9‡ ‡ Coffinite-G (-5.5 ± 3.5) −2,001.1 ± 3.8 ‡ 24.1 ± 3.5 −1,971.5 ± 3.8 ‡ 23.9 ± 4.0 −1,971.7 ± 4.2

*Calculated with auxiliary data from ref. 48. † This value was calculated. ‡Calculated with auxiliary data from ref. 48. §The entropy value was taken from averaging reference data (22, 37), and then was used to calculate the enthalpy of formation.

Guo et al. PNAS Early Edition | 3of5 Downloaded by guest on September 27, 2021 μ radiolysis of water to form H2O2 (25, 51–53); the localized oxi- accelerating voltage, 10-nA beam current, and a spot size of 1 m). Samples dative conditions would help dissolve uraninite into more soluble were pelletized, polished, and carbon coated before analysis. The de- 2+ composition products of coffinite-G were recovered after DSC-TG and the uranyl (UO2 ) species (47). Organic matter in the natural system (6, 8, 12, 18, 28) could then play an important reducing role to ratio of Si/U was further analyzed. precipitate coffinite as summarized by Deditius et al. (8). A se- High-temperature oxide melt solution calorimetry was conducted using a custom-built Tian-Calvet twin microcalorimeter (52–54). Powdered samples quence of UO2 oxidation by peroxide, transport of U(VI) species were hand pressed into small pellets (∼5 mg) and were dropped from room in aqueous solution, and reduction by organic matter in the pres- temperature into either 30 g of molten 2PbO·B2O3 solvent at 802 °C, or 20 g ence of a silica source could produce coffinite at locations distant of molten 3Na2O·4MoO3 solvent presaturated with 100 mg of SiO2 (56) at from the initial UO2 phase. Under such conditions, USiO4 could 703 °C, each held in Pt crucibles. O2 gas was continuously bubbled through form if its crystallization were kinetically favored over that of UO2. the melt at 5 mL/min to ensure an oxidizing environment and facilitate Thus, the process of coffinitization may involve a sequence of dissolution of samples to prevent local saturation (57). Flushing O2 gas at ∼50 mL/min through the calorimeter chamber assisted in maintaining a reactions: UO2 dissolution under locally oxidizing conditions, transport of the dissolved U(VI) species into more reducing constant gas environment above the solvent and removing any evolved ∼ environments containing dissolved silica, followed by coffinite gases (57). The calorimeter was calibrated using the heat content of 5mg α-Al O pellets (58, 59). Upon rapid and complete dissolution of the sample, precipitation. 2 3 the enthalpy of drop solution ΔHds was obtained. Finally, using appropriate Δ Experimental Methods thermochemical cycles, enthalpies of formation from the oxides Hf,ox were calculated. Coffinite-F and -G samples were prepared by hydrothermal synthesis routes, Previous studies show that silica has a low solubility in 3Na2O·4MoO3 but described elsewhere (33, 35). Because these syntheses routes inevitably have silica-containing samples dissolve, precipitating silica as cristobalite (56). A UO2 and amorphous silica as byproducts, further purification is needed. test of this solvent on the dissolution of zircon structure materials was done HNO3 and KOH solution were used to wash the samples as described in the by dropping ZrSiO4 and HfSiO4 in this setup and yielded consistent results purification protocol proposed by Clavier et al. (54). More synthesis and with experiments done in molten lead 2PbO·B2O3 solvent at 802 °C. purification details are shown in SI Appendix. XRD measurements were performed at room temperature using a Bruker – Κα ACKNOWLEDGMENTS. The authors acknowledge H. P. Brau and X. Le Goff D8 diffractometer with Bragg Brentano geometry (Cu radiation, 40 kV, from ICSM for the TEM images of coffinite-F. Preparation, characterization, – and 40 mA), using a step size of 0.016 0.028 °. Lattice parameters were re- and purification of sample coffinite-F were done at ICSM. Funding for these fined by the Le Bail method (55). experiments was supported by the Nucléaire, Energie, Environnement, DSC-TG measurements were performed with a Setaram LabSys instrument Déchets, et Société Resources program of the CNRS. Preparation, purification coupled with a quadrupole mass spectrometer (MKS Cirrus2) for evolved gas of the coffinite-G sample, and part of the characterization were done at analysis. Coffinite samples (∼10 mg) were placed in Pt crucibles without lids Forschungszentrum Jülich. The calorimetric and characterization work at and heated in airflow (40 mL/min) to 1,000 °C at 10 °C/min. For quantitative University of California, Davis, was supported as part of the Materials Science + of Actinides, an Energy Frontier Research Center funded by the US Depart- analysis of evolved gases MS traces for H O (m/z = 18) and CO (m/z = 44) 2 2 ment of Energy, Office of Science, Office of Basic Energy Sciences under were calibrated by decomposition of calcium oxalate in the same experimental Award DESC0001089. We also gratefully acknowledge the support of the conditions. US Department of Energy through the Los Alamos National Laboratory/Lab- Chemical composition and homogeneity were determined using a Cameca oratory Directed Research & Development Program and the G. T. Seaborg SX-100 electron microprobe with wavelength dispersive spectroscopy (15-kV Institute for this work.

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